Separation and purification of acrylic acid from acetic acid by solvent extraction and azeotropic distillation with a two component solvent-entrainer system



March. 1959 AKIRA YOMIYAMA ETAL 3,433,831 7 I SEPARATION ANDPURIFICATION OF ACRYLIC ACID FROM -ACETIC ACID BY SOLVENT EXTRACTION ANDAZEQTROPIC DISTILLATION WITH A TWO COMPQNENT SOLvENT-ENT'RAINER SYSTEMFiled June 29. 1966 Sheet l 0:2

ZXTRQCTION DEHYDRATING coLuum d COLUMN SOLVENT 1 h SOLVENT' 'QSEBX 9-$EPARATION 4 poLunm HAcr= ACRYLIC ACID FC= FIRST COMPONENT SC= SECONDCOMPONENT' March 18, 1969 AKIRA YOMIYAMA ETAL 3,433,831 SEPARATION ANDPURIFICATION OF ACRYLIC ACID FROM ACETIC ACID BY SOLVENT EXTRACTION ANDAZEOTROPIC DISTI LLATION WITH A TWO vQQMPONCENT SOLVENT-ENTRAINER SYSTEMFiled June 29.1966

FIG. 2

a I Q SOLVENT C C RECOVERY j C COLUMN I C ExrRAc'noN C v C COLUMN C 4SOLVENT SEPARATIONS COLUMNS f d l a=H O+ HAcr+HAc HAc(ANDH O)b=Hhcr+HAc+Fc+SC+H 0 i= HAcr c: RAFFINATE J=FC+ sc+ HA6 (AN 0 H20)d=HAc+H O k=FC+SC Q=FC+ SC HAc ACETIC ACID sc= ssc'ouo COMPONENTRAFFINATE HAc,H O, PC AND sc United States Patent O "ice 7 ClaimsABSTRACT OF THE DISCLOSURE A method for separating acrylic acid from amixture of acrylic and acetic acids by distilling the mixture at apressure of less than 350 mm. Hg in the presence of a polymerizationinhibitor and an entrainer consisting of a first component which is atleast one member of the group consisting of aliphatic and alicyclichydrocarbons of 7 carbon atoms and toluene and a second component whichis at least one member of the group consisting of water, esters of theformula R COOR wherein R is ahydrocarbon radical of 1-3 carbon atoms andR is a hydrocarbon radical of l-4 carbon atoms and nitriles of 3-4carbon atoms; said first and second components being present in a molarratio of 1:0.05-15.

A number of studies have been made on a method for producing acrylicacid by vapor phase oxidation of propylene or acrolein. However, studieson refining already produced acrylic acid have been relatively rare.Generally, in crude acrylic acid produced by the aforesaid vapor phaseoxidation acetic acid as by-product is contained in a proportion of fromA to by mol based on the acrylic acid. Accordingly, from an industrialpoint of view the separation of acetic acid from acrylic acid containingthe same is important for producing pure acrylic acid. As regards such aprocess, only several processes have hitherto been proposed includingBritish Patents Nos. 997,324, 997,325, etc. Those processes comprisecooling the product obtained by vapor phase oxidation of propylene oracrolein, absorbing same in water to obtain an aqueous solution ofacrylic acid containing acetic acid and evaporating water therefrom.Alternatively, the aqueous solution of acrylic acid is subjected toextraction by adding a suitable solvent thereto to extract acrylic acidas well as acetic acid and the resulting extract is distilled toseparate the solvent from acrylic acid containing acetic acid and thenacetic acid is removed therefrom by distillation under reduced pressureto obtain pure acrylic acid.

Another proposed process involves adding a suitable solvent capable ofextracting only acrylic acid to an aqueous solution of acrylic acid.This method, however, requires a large amount of solvent and there is aconsiderable loss of acrylic acid accompanied by the loss of thesolvent.

In the aforesaid common distillation, the specific volatilities ofacrylic acid and acetic acid are so small that a large number of platesis required in the distilling column and the reflux ratio must beincreased. This is disadvantageous in practical application on anindustrial scale. Moreover, since acrylic acid is easily polymerized,the separation is generally carried out in the presence of apolymerization inhibitor. Even though, so-called pop- 3,433,831 PatentedMar. 18, 1969 corn polymerization often occurs, and causes a blockage inthe distilling column. This means that the greater the number of platesthere are, the less advantageous is the process. In the distillingcolumn it is desirable to decrease the number of plates where acrylicacid is present in a high concentration but in a binary system ofacrylic acid and acetic acid, the number of plates necessarily increasesdue to the relation between the specific volatilities of acetic acid andacrylic acid and their concentrations.

After thoroughly studying the above distillation, the present inventorshave succeeded in obtaining pure acrylic acid with good efiiciency byadding an entrainer which forms an azeotrope with acetic acid.

That is, it has now been found that acetic acid can readily be separatedfrom acrylic acid by distillation when an entrainer capable of formingan azeotropic mixture with acetic acid is present. The entrainercomprises two groups of components. The first group of componentsincludes aliphatic or alicyclic hydrocarbons having 7 carbon atoms ortoluene or a mixture thereof. The second group of components includeswater, esters of the chemical formula R COOR wherein R is a hydrocarbonradical having 1-3 carbons and R is a hydrocarbon radical having 14carbons, whose boiling point does not exceed C., nitriles having from 2to 4 carbon atoms, and any mixtures thereof. The first component alonemay be used but the effect can be further increased by the addition ofthe second component.

When the first component only is used as an entrainer in the azeotropicdistillation, the first component forms an azeotropic mixture withacrylic acid even though in a small amount. Accordingly, a part of theacrylic acid will necessarily be distilled out when distilling aceticacid. However, when the second component is added to the firstcomponent, substantially no acrylic acid enters the acetic aciddistillate. The mixing ratio of the first component to the secondcomponent is 1 mol of the former to 0.05-1.5 moles of the latter,preferably 0.2-1.5.

The first component is selected so as to form an azeotropic mixture withacetic acid but not with acrylic acid, and n-heptane is desirable as thealiphatic hydrocarbon of seven carbon atoms. In the case of the firstcomponent, when the number of carbon atoms is less than 6, an azeotropicmixture with acetic acid is not formed and on the other hand, when thenumber of carbon atoms is more than 8 the separation of acetic acid fromacrylic acid becomes difiicult, and therefore neither case is suitable.

The amount of the second component to be added is required to be notless than 0.05 mol per mol of the first component but when it exceeds2.0 moles it restrains the action of the first component, with theresult that acetic acid is hardly distilled out at the head of thecolumn. It is therefore, preferable to use from 0.2 to 1.5 moles.

Ketones and ethers have a similar effect as the second component, butare not preferable because acrylic acid is not stable and easilypolymerizes in the presence of such compounds. Accordingly, from anindustrial point of view, ethyl acetate, methyl acrylate, propylacetate, ethyl acrylate, acetonitrile, acrylonitrile and water arepreferable as the second component.

In eifectin-g azeotropic distillation by the use of the entrainerconsisting of the first and second components, in order to avoid thepolymerization of acrylic acid, it is necessary to carry out thedistillation under a reduced pressure of less than 350 mm. Hg abs. withthe use of a known polymerization inhibitor, for example, hydroquinone,hydroquinone monomethyl ether, phenothiazine, oxygen, etc.

As for the method of introducing the entrainer, it may be introducedinto the column from the head as in the case of usual azeotropicdistillation or into an enriching zone of the column or into thestarting feed. Alternatively, a part or all of the first component maybe mixed into the starting feed and a part or all of the secondcomponent from the head of the column, i.e., not only from one regionbut also from two or more regions.

In the distillation according to the present method the distillate doesnot always form two liquid layers as is often seen. When the distillateazeotrope does not form two liquid layers, acetic acid may be removedtherefrom by extraction with water or acetonitrile or neutralization oresterification or by other known methods. The remaining liquor fromwhich acetic acid has been removed can be used again as an entrainer.

In the process for the production of acrylic acid by the vapor phaseoxidation reaction of propylene or acrolein, usually not more than 40%by weight, preferably 30% of a crude aqueous acrylic acid solution isobtained and it contains an amount of acetic acid from A to 1 (molratio) based on the acrylic acid. A very convenient process can beestablished when the above aqueous solution is processed by acombination of the azeotropic distillation and the extraction.

That is, the crude acrylic acid solution containing acetic acid iscontacted beforehand with a mixed solvent consisting of the first andthe second components excluding water-soluble components, for example,water, aceto nitrile and methyl acetate, to extract acrylic acid fromthe crude solution and then the extract is distilled, thereby the aceticacid as well as the solvent can be distilled out from the top of thedistillation column while acrylic acid free from acetic acid is obtainedfrom the bottom. The distillate may if desired by recycled after aceticacid is taken off. Even when the solvent containing acetic acid isreused, the acetic acid does not accumulate too much in the contactingstep and is discarded from the contacting system as raftinate.

In the composition of the thus obtained extract to the distilling column(hereinafter referred to as the solvent separation column) where thesolvent is to be removed, the mol ratio of the second component to thefirst component is preferably from 0.2 to 1.5 to 1. Therefore, when thesolvent is introduced into the contacting column (hereinafter referredto as the extraction column) where acrylic and acetic acid areextracted, the mol ratio of the second component to the first componentin the extraction solvent is naturally determined so as to come from 0.2to 1.5 to 1. In the composition of the solvent thus determined, theextraction of acrylic acid becomes insulficient in some cases accordingto the kind of solvent used. In such a case, the insufiiciency ofextraction can be overcome by increasing the amount of the secondcomponent, which is hardly soluble in water. However, difiiculty oftenoccurs in distilling out all of the acetic acid as overhead from thesolvent separation column when the proportion of the second component inthe composition of the feed of the solvent separation column increasestoo much. In such a case, a dehydrating tower is placed between theextraction column and the solvent separation column to distil off a partor all of the water and/ or a part of the solvent so as to make the molratio of the second component to the first component in the dehydratingtower equal to 0.2 to 1.5 to 1, and then the resulting bottoms are fedto the solvent separation column.

In the drawing, FIGURE 1 shows one embodiment of the present inventionand FIGURE 2 shows another embodiment thereof. FIG. 2 illustrates theprocess wherein the extract b is fed directly to the solvent separationcolumn as described below in Example 6.

An aqueous acrylic acid solution containing acetic acid is introducedfrom a into the top of an extraction column 1, and an extraction solventsubstantially containing the first component, the second component and asmall amount of acetic acid is introduced countercurrently from 7. Anextract containing acrylic acid and acetic acid is obtained from b. Theweight ratio of the feed liquor from a to the extraction solvent from fis 1:0.6-5, preferably 1108-3.

The extract liquor is dehydrated at a dehydrating tower 3 and then fedinto a solvent separation column 4 through g. The dehydrating tower maybe operated under normal pressure or reduced pressure and when thedistillate forms two liquid layers upon condensation, only an oily layeris refluxed to the tower, and thus the dehydration can be readilycarried out. The solvent separation column is preferable to carry outthe distillation under a reduced pressure of less than 350 mm. Hg abs.in order to avoid the polymerization due to an increase in theconcentration of acrylic acid in the solvent separation column. From 1'of the solvent separation column 4, acrylic acid substantially free fromacetic acid is obtained. The distillate j of solvent separation column4, the distilulate h from the dehydrating tower 3 and recovered solvente from the solvent recovering column 2 and a make up k for solvent lossmay be combined as the extraction solvent f. The recovered solvents maybe returned to the extraction column 1 combining all together orindividually. As to the behavior of acetic acid contained in the solventintroduced from 1, when the operation of the processes achieves a steadystate, a fixed amount of acetic acid is constantly recycled from b to fpassing through j, whereby the removal of acetic acid contained in thefeed liquor can be continuously carried out, because the same amount ofacetic acid is contained in the residue as in the feed of the extractioncolumn. The residue of the extraction column is introduced from c intothe solvent recovery column 2, thereby recovering the solvent containedin said liquor. A diluted aqueous solution of acetic acid is obtainedfrom d and acetic acid can be recovered therefrom by any known method.

In order to prevent the polymerization of acrylic acid in each step ofthe aforementioned process, a known polymerization inhibitor, forexample, hydroquinone, phenothiazine, oxygen, etc., is used.

Any type of the extraction column 1 may be used, for example,mixer-settler, packed column, perforated plates column or a columnhaving turning blades and pulse generating devices are all usable.

Furthermore, the continuous layer of the extraction column may be eithera water layer or organic layer. The extraction temperature is preferablyfrom 0 to 60 C. In some cases, a three phase liquid is formed accordingto the kind of the first component and the second component which ishardly soluble in water, the mixing ratio thereof and the concentrationof acid, etc. In such a case, a two phase liquid is convenientlyobtained by elevating the temperature or replacing a part, i.e., lessthan three tenths of the first component with benzene, so that the zonewhich forms the three phase liquid is narrowed.

The following examples are given to illustrate the invention but theyare not to be construed as limitative of its scope.

EXAMPLE 1 Solutions consisting of ethyl acetate, n-heptane, acetic acidand acrylic acid, and mixtures thereof to which water is added, were fedwith a constant feed pump at a rate of 20 g./hr. into the middle of atriple tubular type glass distilling column of 2000 mm. high and of 20mm. in diameter (packed with coiled wires of stainless steel SUS 27 of3.5 mm. in diameter and a length of 3.5 mm. of the trade name NaniwaPack No. 3).

The distillation was continuously carried out under operating pressureof mm. Hg, with a reflux ratio of 1.1, and a column bottom temperatureof 99101 C. and a column head temperature of 41 -45 C.

The distillations of feeds having various compositions were carried outand the results obtained were as shown in the following table.

TABLE Composition (percent by weight) Experiment No.:

poopcopcopoopocpoopoopccpoo b/a. weight Molratloinb ratio) EtAc/n-Hepance Wherein, a is the feed, b is the distillate, c is column bottomliquor, EtAc is ethyl acetate, n-Hep is n-heptane, HAc is acetic acid,and HAcr is acrylic acid.

As is clear from the above table, the ease of separation of acetic acidand acrylic acid varies according to the mol ratio of ethyl acetate ton-heptane. As shown in Experiment 5, in case of ethylacetate/n-heptane=1.7 (mol ratio), a considerable amount of acetic acidremains at the column bottom, but by making this ratio smaller, aceticacid does not remain at the column bottom and it distils out from thecolumn head, and conversely acrylic acid is not contained in thedistillate. Such an ideal or optimum ratio of ethyl acetate/n-heptanemay considerably be atfected by the concentrations of acrylic acid andacetic acid but in the present examples, a ratio in the range of 0.2-1.5to 1 is considered suitable and in this case 99% of acetic acid isremoved from the column head without any acrylic acid loss.

If the ratio is further reduced, as shown in Experiment 6, acrylic acidis contained in the distillate liquor in a large amount.

Experiments 7-9 show the cases where water is present.

As shown in Experiments 7-9, a mol ratio of ethylacetate and water ton-heptane in the range of 0.5-1.5 to 1 is also considered to besuitable.

Furthermore, this example and subsequent examples were carried out withthe addition of 0.2% of hydroquinone monomethyl ether as apolymerization inhibitor.

Under the same conditions as in this example, the direct separation ofacrylic acid and acetic acid was carried out without using n-heptane andthe following results were obtained.

Composition (percent by weight) Flow rate Acrylic Acetic High heavy acidacid substances In the above case, it is obvious that the separation isincomplete, compared with that of the azeotropic distillation.

EXAMPLE 2 The separation of acetic acid and acrylic acid was carried outby the azeotropic distillation with the same distilling column made ofglass as in the Example 1, using n-heptane as entrainer.

The distillation was continuously carried out under a pressure of 175mm. Hg while feeding the feed liquor consisting of 84% of acrylic acidand 16% of acetic acid into the column at a position /3 from thebottomof the column with a constant feed pump at a flow rate of 25.0g./hr., maintaining the column bottom liquor at 10l C. and addingn-heptane thereto from the reflux inlet at the rate of 13.4 g./hr.

The distillate liquor was countercurrently washed with water at the rateof 8.8 g./hr. with a two-stage mixer settler and there was obtained washwater of the following composition at a rate of 13.7 g./hr.

Percent Acryl1c acid 6.0 Acetic acid 2.9.3 Water 63.2 n-Heptane 1.5

The water washed layer was almost completely com- Percent Acrylicacid 1. 98.4 Acetic acid Trace n-Heptane 0.6 Other heavy substances 0.6

Thereby, the yield of acrylic acid was 95.5%.

On the occasion of the afore-mentioned azeotropic distillation, in thesame way as in Example 2 the azeotropic distillation was carried out butusing n-heptane as an entrainer added thereto at a rate of 20.0 g./hr.,simultaneously, together with water at a rate of 0.54 g./hr. and as aresult, the following result was obtained.

Flotw Composition (percent by weight) ra e (g./hr.) Acrylic Aceticn-Heptane Water 2101 acid Feed 25.0 84.0 16.0 Water washing 13.1 0.830.6 1.5 67.2 Bottom distillate n... 21.0 99.1 Trace 0.5

1 Obtained by washing the distillate with water at a rate of 8.8 g./h1'.

Besides those mentioned herein, 0.6% of high boiling point substanceexisted.

7 In the above case, the recovery rate of acrylic acid was 99.1%, and itis therefore, obvious that if the distillation is carried out byallowing water to be present in the optimum amount, the amount ofacrylic acid in the distillate is reduced, thereby improving the effectof the separation.

EXAMPLE 3 The azeotropic distillation was carried out using n-heptane asan entrainer in the presence of acetonitrile under a pressure of 175 mm.Hg with the same distilling column made of glass as in Example 1.

N-heptane saturated with acetonitrile (containing 2.5% of acetonitrile)was added thereto at a rate of 15.9 g./hr. from a reflux inlet at thetop of the column.

The distillation was continuously carried out while feeding a solutionconsisting of 10.8% of acetic acid and 89.2% of acrylic acid at a rateof 34.8 g./hr. into the column at the /3 position from the columnbottom, and as a result, a column bottom of the following compositionwas obtained at a rate of 30.4 g./ hr.

Percent Acrylic acid 99.3 Acetic acid Trace n-Heptane 0.3 Other heavysubstances 0.4

The distillate liquor was washed with acetonitrile containing 9.1% ofn-heptane at a rate of 11.1 g./hr. to obtain a washing liquor of thefollowing composition at a rate of 14.8 g./hr.

Percent Acrylic acid Trace Acetic acid 25.0 n-Heptane 6.7 Acetonitrile68.3

The washing liquor was distilled by another distillation column undernormal pressure to obtain acetic acid from the column bottom. Thedistillate liquor was acetonitrile containing 9.1% of n-heptane and wasrecycled again as washing liquor. The liquor washed with water wasn-heptane saturated with acetonitrile containing hardly any acid and wasrecycled as an entrainer of the azeotropic distillation column.

In the persent example, acrylic acid was refined with 99.3% yield,recovering high purity acrylic acid from the column bottom as well ashardly distilling out acrylic acid from the column head.

EMMPLE 4 The azeotropic distillation was carried out using toluene as anentrainer in the presence of ethyl acrylate with the same distillingcolumn as in Example 1.

The entrainer consisting 25.6% of ethyl acrylate, 72.7% of toluene andabout 1.1% of water saturated therewith was added to the column at arate of 18.0 g./hr. under a pressure of 175 mm. Hg from a reflux inletat the top of the column.

The feed consisting of of acetic acid and 90% of acrylic acid was fed ata rate of 25.0 g./hr. into the column at the /3 position from the columnbottom. The distillation was continuously carried out to obtain a columnbottom liquor of the following composition at a rate of 22.5 g./hr.

Percent Acrylic acid 99.1 Acetic acid Trace Toluene 0.4 Other heavysubstances 0.5

The distillate was countercurrently washed with water at a rate of 6.3g./hr. with a two-stage mixer-settler and the resulting washing water ofthe following composition was obtained at a rate of 8.8 g./hr.

Percent Acrylic acid Trace Acetic acid 27.3 Water 71.4 Ethyl acrylate1.2 Toluene Trace The water washed liquor was returned to the columnagain as an entrainer from the top of the column while supplementing acorresponding amount of ethyl acrylate to that amount which had beentransferred into the washing water.

In the present example, highly pure acrylic acid was obtained from thecolumn bottom with a recovery rate of 99.1%, and at the same timeacrylic acid was hardly distilled out from the column head.

EXAMPLE 5 As the extraction column illustrated in the drawin packedcolumn 270 cm. in height and 1.0 cm. diameter (packed with the samepacking as in Example 1) was used. The extraction was countercurrentlycarried out while pouring an aqueous solution containing 25% by 'weightof acrylic acid (hereinafter all percent referred to are percent byweight) and 4% of acetic acid as the feed liquor into the column fromthe column head with a constant feed pump at a rate of 13.6 g./hr.,quantitatively introducing an extraction solvent consisting of 59.5% ofethyl acetate, 39.6% of n-heptane and 0.9% of water into the column fromthe column bottom at the rate of 19.0 g./hr., and maintaining the liquidat 30 C.

As a result, an extract of the following composition was obtained at arate of 23.0 g./hr.

Percent Acrylic acid 14.7 Acetic acid 1.7 Ethyl acetate 47.1 n-Heptane32.6 Water 3.9

Flgw Composition (percent by weight) ra e g./hi Acrylic Acetic WaterEthyl n-Heptane acid acid acetate Distillate 10. 6 0. 68. 8 22. 7

Bottom- 12. 4 27. 8 3. 1 0. 05 28. 6 41. 0

The distillate fonms two liquid phases, and the ratio of organic layerto water-layer is about 14 to 1.

Next, the tower bottom liquor of the dehydrating tower was fed into themiddle of the solvent separation column as shown in the drawing and theseparation of acrylic acid and the other components was carried out. Asthe solvent separation column, the same type of column as thedehydrating tower was used, but it was 200 cm. high.

The distillation was continuously carried out while introducing the feedwith a constant feed pump into the middle of the column at a rate of12.4 g./ hr. with the reflux ratio of 0.8, under an operating pressureof 175 mm. Hg and at a column bottom temperature of C. and as a result,the following was obtained.

F1 Composition (percent by weight) ow rate, Acrylic Acetic Ethyln-Heptane Other heavy g./hr. acid acid acetate substance Distillate 9.4. 2 39. 3 66. 4

Bottom Thus, acrylic acid containing a very small amount of acetic acidwith recovery yield of 99.3% is obtained from an aqueous solution ofacrylic acid.

Next, as shown in the flow sheet of the drawing, when the portion oforganic layer of the distillate of the dehydrating tower and therecovered solvent from the Flow rate, g./hr.

Composition (percent by weight) Acrylic Acetic n-Heptane Ethyl Wateracid acid acetate rafiinate of the extraction column by distillation(sub- Distillam" 2L2 L3 491) 48.0 L7 stantially ethyl acetate wrth atrace of n-heptane) and the tto 3.01 98.9 0.3 0.05 0.3 organic layer ofthe distillate of the solvent separation column which contained aceticacid were combined to- Furthermore, the column bottoms Contained 0fgether, with ethyl acetate and n-heptane being added in high boilingPoint Substanceamounts corresponding to those which had been lost AS canbe een from the above, acrylic acid containduring the operations wereadded respectively thereto and ihg a y Small amount of acetic c d WasObtained t it was recycled as the extraction solvent, the concentrationrecovery yield of 987% from an aqueous thereof acetic acid in theextract increased slightly to 2.1% but the composition of the columnbottoms of the solvent EXAMPLE 7 separation column was not affected bythe recycling of Said Solvent To the column head of the same extractioncolumn as Furthermore, this example and subsequent example P 5, as the eliquor an eq e Solution were carried out with the addition of 0.2% ofhydrotalmPg by Welght of acryllc acld and 41% of quinone monomethylether as the polymerization inhibitor. acetlc 391d at a rate of and wastracted at the liquor temperature of 30 C. with a mixed EXAM L 6 solventconsisting of 53.5% of ethyl acetate, 40.1% of n-heptane and 4.4% ofbenzene at the rate of 19.7 g./hr. An aqueous solution containing 20.0of acrylic acid and then was dehydrated with the dehydrating tower ofand 2.67% of acetic acid was introduced at a flow rate of Example 5under normal pressure with the reflux ratio 15.0 g./hr. into the sameextraction column as in Example of 0.3. The tower bottoms of thedehydrating tower were 5 through the column head and a mixed solvent, ofwhich distilled in the same solvent separation column as in the weightratio of n-heptane to ethyl acrylate was 1 to Example 5 under a pressureof 175 mm. Hg. with the 1, containing saturated water therewith wasadded at a reflux ratio of 0.7, and as a result, the following was rateof 21.0 g./hr. from the column bottom. The liquidobtained. d

Flgw Composition (percent by weight) 2713. Acrylic Acetic Water Ethyln-Heptane Benzene Other heavy acid acid acetate substances liquidextraction was carried out at 30 C. and as a result, From an aqueoussolution, acrylic acid was obtained an extract liquor of tht followingcomposition was obin 99.1% yield. tained from the column head at a flowrate of 24.4 g./hr. EXAMPLE 8 Percent To the column head of the sameextraction column Acrylic acid 12.3 as in Example 5, as the feed liquor,an aqueous solution Acetic acid 1.1 containing 20.0% by weight ofacrylic acid and 3.1% of Ethyl ta rylate 42.1 acetic acid was fed at arate of 16.8 g./hr., subjected to nJ-Ieptane 42.9 extraction with amixed solvent consisting of 59.1% of W t 1.5 acrylonitrile, 39.4% oftoluene and 1.5% of water at a rate of 19.2 g./hr. and at a liquortemperature of 40 C. Thus, in this extraction column, acrylic acid andacetic and then the extract Was dehydrated with the dehydratacidcontained in an aqueous solution were extracted in 8 tower of Example 5under normal Pressure with the 99.3% and yield, respectively. refluxratio of 0.1. The tower bottom liquor of the de- The extract liquor wasdirectly fed into the same 501- hydrating towel was then distilled illthe Same Solvent vent separation column as in Example 5 and thedistilla- 65 Separation 5011111111 as in Example 5 under a Pressure 0ftion was carried out under a pressure of 175 mm. Hg., at 140 mm. Hg.with the reflux ratio of 1.2. and as a result, a column bottomtemperature of C. and with the the following was obtained.

Flow Composition (percent by weight) 57%;. Acrylic Acetic WaterAcrylonitrlle Toluene Other heavy acid acid su stances From an aqueoussolution, acrylic acid scarcely containing acetic acid was obtained in98.3% yield.

We claim:

1. A method for separating acrylic acid from a mixture of acrylic acidand acetic acid by azeotropic distillation, said method comprisingdistilling the mixture at a pressure less than 350 mm. Hg in thepresence of a polymerization inhibitor and an entrainer consisting of afirst component which is at least one member selected from the groupconsisting of aliphatic and alicyclic hydrocarbons having 7 carbonatoms, and toluene and a second component which is at least one memberselected from the group consisting of water, an ester of the formula: RCOOR wherein R is a hydrocarbon radical of 1 to 3 carbon atoms and R isa hydrocarbon radical of 1 to 4 carbon atoms, said ester having aboiling point of at most 130 C., and nitriles of 3 to 4 carbon atoms;said first and second components being present in a molar ratio f1:0.05-1.5.

2. A method according to claim 1, wherein the mixture of acrylic acidand acetic acid further comprises water.

3. A method according to claim 1, wherein up to three tenths of themolar amount of the first component is replaced by benzene.

4. A method for separating acrylic acid from an aqueous solution ofacrylic and acetic acids, said method comprising extracting acrylic andacetic acids from the aqueous solution with a solution consisting of afirst component which is at least one member selected from the groupconsisting of aliphatic and alicyclic hydrocarbons having 7 carbonatoms, and toluene and a second component which is at least one memberselected from the group consisting of water, an ester of the formula: RCOOR wherein R, is a hydrocarbon radical of 1 to 3 carbon atoms and R isa hydrocarbon radical of 1 to 4 carbon atoms, said ester having aboiling point of at most 130 C., and nitriles of 3 to 4 carbon atoms,said first and second components being present in a molar ratio of'1:0.051.5, to obtain an extract, subjecting the thus obtained extractto azeotropic distillation to obtain acrylic acid as a bottom and amixture consisting of acetic acid, the first component, the secondcomponent and water as a distillate, and recycling the distillate to thee xtracting step.

5. A method according to claim 4, wherein up to three tenths of themolar amount of the first component is replaced by benzene.

6. A method for separating acrylic acid from an aqueous solution ofacrylic and acetic acids, said method comprising extracting acrylic andacetic acids from the aqueous solution with a solution consisting of afirst component which is at least one member selected from the groupconsisting of aliphatic and alicyclic hydrocarbons having 7 carbonatoms, and toluene and a second component which is at least one memberselected from the group consisting of water, an ester of the formula: RCOOR wherein R is a hydrocarbon radical of 1 to 3 carbon atoms and R isa hydrocarbon radical of 1 to 4 carbon atoms, said ester having aboiling point of at most C., and nitriles of 3 to 4 carbon atoms toobtain an extract, subjecting the extract to distillation to remove atleast a portion of the water and portions of the) first and secondcomponents from the extract, to thereby regulate the molar ratio of thefirst component to the second component to a range of 1:0.05-1.5 andobtain a bottom, subjecting the thus obtained bottom to azeotropicdistillation to separate acrylic acid as a bottom therefrom and amixture consisting of acetic acid, the first component, the secondcomponent and water as a distillate and recycling the distillate to theextracting step.

7. A method according to claim 6, wherein up to three tenths of themolar amount of the first component is replaced by benzene.

References Cited UNITED STATES PATENTS 1,668,380 5/1928 Ricard 203-151,860,553 5/1932 Ricard et al. 203-51 1,915,002 6/1933 Ricard et al.203-15 2,854,385 9/1958 Alheritiere 203-16 3,337,740 8/1967 Gray et al.203-69 3,344,178 9/1967 Brown et al. 203-15 WILBUR L. BASCOMB, JR.,Primary Examiner.

US. Cl. X.R.

